Inductive elements (also referred to as electromagnetic elements) are a group of circuit components that take advantage of various electromagnetic properties and are commonly used in a wide-range of integrated circuit applications. Transformers, for example, are used to transfer electrical energy from one circuit to another circuit through a shared magnetic field. The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism), and second, that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). By changing the current in the primary coil, the strength of the magnetic field is changed. Since the secondary coil is wrapped around the same magnetic field, a voltage is induced across the secondary coil. By adding a load to the secondary circuit, one can make current flow in the second circuit, thus transferring energy from one circuit to the other circuit.
FIGS. 1 and 2 illustrate schematic views of a simplified transformer design and circuit. In operation, a current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, which ensures that most of the magnetic field lines produced by the primary current are within the iron and pass through the secondary coil as well as the primary coil.
Increasing transformer efficiency and reducing size is important for circuit design and integration. One technique for increasing efficiency and reducing size is to integrate a magnetic material, such as a magnetic film (e.g., a ferromagnetic film such as CoFe, CoFeB, NiFe, etc.), on either end of the metal turns. The magnetic film can enhance the magnetic flux density B, which greatly increases the permeability and the electromotive force (EMF) of the transformer. Put another way, for a given EMF of the transformer, the integration of a magnetic film can reduce the size of the transformer and/or improve the transformer efficiency.
Such transformers and other inductive elements can be integrated into a logic/RF CMOS process by utilizing standard CMOS back-end process steps, such as metal deposition, dielectric deposition, and metal patterning in the CMOS foundry. However, magnetic films conventionally require advanced processing techniques that are more difficult to implement.